Light emitting diode (LED) lamps have been developed to replace conventional incandescent or fluorescent lamps to reduce electrical and maintenance costs and to increase reliability. LED lamps consume less electrical energy than conventional lamps while exhibiting much longer lifetimes. Such lamps are typically powered by a switching power supply, which provides a substantially constant output voltage even with large changes in the input voltage and the ambient temperature.
One popular application for LED lamps is in traffic signals. LED lamps are used to replace conventional 8 and 12 inch round signs, pedestrian signs, hand signs, arrow signs and signs with messages used in traffic signals. Such LED lamps typically include a switching power supply that operates over an input voltage range of 85-130 VAC, while producing a substantially constant output voltage to operate the LEDs. The switching power supply also senses a "fail state" situation, where more than a predetermined number of LEDs have failed (burned out). When a "fail state" is detected, the power supply for the LED lamp shuts down and a signal is sent to the traffic maintenance unit for repair.
LED traffic lamps are exposed to widely changing climate conditions. Therefore, agencies like the Institute of Transportation Engineers have developed output specifications for LED traffic lamps. These specifications call for the LED traffic lamps to provide a minimum specified light output throughout an ambient temperature range of -40.degree. C. to +74.degree. C. While typical switching power supplies can supply a fixed output voltage to the LED lamp throughout the specified temperature range, there are several temperature induced problems that may cause LED lamps to fail to meet the light output specifications.
The first such temperature induced problem occurs with low ambient temperatures. As the ambient temperature of the LEDs drop down toward -40.degree. C., the electrical resistance of the LEDs rises (forward voltage rises, where the forward voltage is the voltage required across the LEDs to pass a predetermined current through the LEDs), thus causing the operating current to drop. The lower operating current causes an undesired drop in the light output level from the LEDs, possibly even below the minimum specified level.
The second temperature induced problem occurs with high ambient temperatures. As the ambient temperature rises toward +74.degree. C., the efficiency of the LEDs drops, causing the light output level to drop even though the drive current stays relatively constant.
The third temperature induced problem relates to the detection of the "fail state" condition. Conventional switching power supplies utilize a transistor to turn off the power to the LEDs when the fail state condition occurs (i.e. more than a predetermined number of LEDs are burned out). This circuitry senses the overall current through the LEDs. If the LED drive current drops below a certain level, the turn-on voltage to the transistor is reduced to the point that it shuts off, thus shutting off power to the LEDs. The problem with this design, however, is that the turn-on voltage level needed to turn the transistor on and off varies with temperature. Thus, the "fail state" function of turning off the LED lamp when a predetermined number of LEDs are burned out does not function consistently for different ambient temperatures.
There is a need for an LED lamp that provides relatively constant light output at low and at high temperatures. There is also a need for such an LED lamp to consistently turn itself off when a predetermined number of LEDs are burned out, where the predetermined number does not change significantly with changes in ambient temperature.
There are a number of conventional temperature compensation circuit designs that use sensors (U.S. Pat. No. 5,818,225, U.S. Pat. No. 5,640,085), FET variable resistors (U.S. Pat. No. 5,397,933) strain gauge pressure sensors (U.S. Pat. No. 5,616,846), and pulse frequency/width adjustment (U.S. Pat. No. 5,783,909, U.S. Pat. No. 5,886,564). However, these temperature compensation schemes are complex and expensive.